1. Technical Field
The present invention relates to a liquid crystal display device. More particularly the invention relates to a liquid crystal display device in which bright point defects scarcely occur during manufacturing, and in addition, an auxiliary capacitance is increased without a reduction in the aperture ratio of each pixel, thereby making it possible to achieve a good display picture quality with little flicker and cross talk even in a liquid crystal display device having pixels that have comparatively small pixel areas or that are made high-definition, and to a method for manufacturing the same.
2. Related Art
In recent years, liquid crystal display devices have been commonly used not only in information communication devices but also in general electrical equipment. Such a liquid crystal display device comprises the following elements: an array substrate, which forms on its surface a matrix of scan lines and signal lines, and in the areas enclosed by these lines, forms thin film transistors (hereinafter called TFTs) acting as the switching elements for driving liquid crystal, pixel electrodes that apply voltage to the liquid crystal, and an auxiliary capacity formed by auxiliary capacitor lines and auxiliary capacitor electrodes to hold signals; an opposed substrate, which forms on its surface the color filter layers of red (R), green (G), and blue (B) as well as a common electrode and so on; and liquid crystal filled between the two substrates.
The auxiliary capacitor electrodes that are formed on the array substrate are provided to form an auxiliary capacity that holds for a certain period of time the electric charge of the signals supplied from the signal lines. The auxiliary capacity is provided by forming an auxiliary capacitor that uses as electrodes the auxiliary capacitor electrodes and part of the TFT's drain electrode or pixel electrode, and as a dielectric body the gate insulating film that covers the TFT's gate electrode. The auxiliary capacitor lines and auxiliary capacitor electrodes are generally formed from a light-blocking conductive material such as aluminum, molybdenum, or chromium. In addition, as shown, for example, in FIG. 9, which is a schematic plan view of a pixel in a liquid crystal display device, it is a common practice that an auxiliary capacitor line 51 and an auxiliary capacitor electrode 52 are provided separated from a TFT 53 and located in the central part of each pixel.
From the viewpoint of preventing flicker and cross talk in the liquid crystal display device, the auxiliary capacity needs to be made large in its capacitance. However, the size of the individual pixels has been reduced as the liquid crystal display device has become smaller and higher-definition with the technological innovations of recent years. Therefore, in a liquid crystal display device 50 in which the auxiliary capacitor line 51 or the auxiliary capacitor electrode 52 is arranged as shown in FIG. 9, it is practically difficult to adopt the idea of thickening the auxiliary capacitor line 51 or the auxiliary capacitor electrode 52 to increase the auxiliary capacitance when considering the aperture ratio of each pixel, because the auxiliary capacitor line 51 and the auxiliary capacitor electrode 52 block light.
As a method to resolve the above problem, there is disclosed an invention of a liquid crystal display device in which an auxiliary capacitance is made larger than that of conventional examples, without changing the size of an auxiliary capacitor electrode. The array substrate in a liquid crystal display device 90 that is disclosed in this invention of Japanese Patent No. 2584290 will be described using FIG. 10 and FIG. 11. FIG. 10 is a plan view of several pixel portions of a array substrate disclosed in Japanese Patent No. 2584290, and FIG. 11A to FIG. 11G are partial cross-sectional views showing in sequence the process of manufacturing the array substrate of FIG. 10.
First of all, an auxiliary capacitor line 92 made of indium tin oxide (ITO) or indium zinc oxide (ITO) is pattern-formed over an insulating substrate 91 made of glass plate. Then a gate metal film 93 is formed and patterned (FIG. 11A). Furthermore, by means of plasma CVD or other, an insulating film 94 made of SiNx or SiOx, an amorphous semiconductor film 95 made of for example a-Si as an active layer, and a semiconductor film 96 for ohmic contact made of for example n+a-Si film doped with impurities, are formed one after another (FIG. 11B). At this point, the thickness X of the insulating film is made sufficiently thick, for example as X=4000 Å, to ensure that no short circuit will occur between the drain and gate or between the source and gate.
Next, the semiconductor film 96 for ohmic contact and the amorphous semiconductor film 95 are etched into patterns with the same resist (FIG. 11C). Then, the resist (not shown in FIG. 11), which remains as aperture patterns (broken-line portions in FIG. 10) at the portions where the auxiliary capacitor line 92 overlaps a pixel electrode 97 to be formed in the subsequent process, is coated, and by means of the etchant for the insulating film 94, such portions are etched to be thinned to the intended thickness Y=2000 Å to serve as the auxiliary capacitor insulating film (FIG. 11D).
Next, the pixel electrode 97 made of ITO is formed and patterned (FIG. 11E). Then a metal film 98 for drain and source is also formed and patterned (FIG. 11F), and the semiconductor film 96 for ohmic contact that is left on a TFT channel portion is etched away, whereupon the array substrate for the liquid crystal display device is completed (FIG. 11G). The liquid crystal display device 90 is then obtained by placing opposite to each other the array substrate structured as above and a common electrode substrate, with liquid crystal substance interposed between them.
In such a related-art technology, the auxiliary capacitor line 92 and pixel electrode 97 are equivalent to the electrodes of a capacitor, and the insulating film 94 located between the auxiliary capacitor line 92 and pixel electrode 97 is equivalent to the dielectric body of a capacitor. Because the thickness X of the insulating film 94 over the gate metal film 93 is 4000 Å, whereas the thickness Y of the insulating film over the auxiliary capacitor line 92 is made to be 2000 Å, this technology has the advantages that short circuits are unlikely to occur between the drain and gate or the source and gate, and moreover that the required auxiliary capacitance can be secured even if the area of the auxiliary capacitor line 92 is not enlarged.
However, in the array substrate of the liquid crystal display device 90 disclosed in Japanese Patent No. 2584290, the surface of the auxiliary capacitor line is partially etched so that the gate insulating film alone is made thin, and thereby the auxiliary capacitance is increased, with electrical insulation being kept unchanged between a gate electrode and scan line covered by the gate insulating film on the one hand, and other members on the other hand. As a result, a larger auxiliary capacitance can be secured compared with the case in which an auxiliary capacitor electrode of conventional examples is used. However, the processing time is increased because the auxiliary capacitor line 92 uses a transparent conductive material such as ITO, which is different from the gate metal film 93. Moreover, there is still a room for improvement in aperture ratio because the area of the auxiliary capacitor line overlapping the pixel electrode is large.
On the other hand, the present inventors found the following method for resolving the problem of the liquid crystal display device 90 disclosed in Japanese Patent No. 2584290, and applied the invention as Japanese Patent Application No. 2006-184115 (hereinafter called “prior application”). A drain electrode of a TFT is extended and used as an electrode pairing with the auxiliary capacitor line, which is an electrode of the capacitor forming an auxiliary capacity, and between the electrode pair is interposed, in place of the gate insulating film, a thinned insulating layer that is made thinner than the gate insulating film. By this method, the capacitance of the auxiliary capacitor can be increased especially without leading to an increase in processing time or reduction in aperture ratio.
The liquid crystal display device shown in the specification and drawings of the prior application will be described using FIG. 12 to FIG. 15. FIG. 12 is an enlarged plan view of a portion corresponding to a single pixel in the liquid crystal display device of the prior invention, viewed through a color filter substrate. FIG. 13 is a cross-sectional side view showing a structure along the cut line XIII-XIII of the liquid crystal display device shown in FIG. 12. In addition, FIG. 14A to FIG. 14G are cross-sectional views showing the process of manufacturing the array substrate illustrated in FIG. 12, and FIG. 15A to FIG. 15E are cross-sectional views showing the process subsequent to FIG. 14G of manufacturing the array substrate illustrated in FIG. 12. All of FIGS. 14 and 15 illustrate the cross-sectional views along the cut line XIII-XIII shown in FIG. 12.
The liquid crystal display device 10D of the prior application is made by bonding the outer circumferential surfaces of a pair of substrates, that is an array substrate 13 and a color filter substrate 14, with each other using a sealing material (omitted in the drawing), and then injecting liquid crystal 15 into the interior portion. The array substrate 13 has transparent substrates 11 and 12 made of glass or other, with various wiring lines, etc. formed thereon.
On the array substrate 13 and color filter substrate 14 (inner surface) there are formed various wiring lines and so on. The array substrate 13 is provided with multiple scan lines 16, signal lines 17, auxiliary capacitor lines 18, thin film transistors (TFTs), and pixel electrodes 20. The scan lines 16 and signal lines 17 are formed into a matrix. The auxiliary capacitor lines 18 are provided between the scan lines 16, parallel to the scan lines 16. The TFT is composed of a source electrode S, a gate electrode G, a drain electrode D, and a semiconductor layer 19 made of amorphous silicon (a-Si), etc. The pixel electrode 20 is provided so as to cover the area enclosed by the scan lines 16 and signal lines 17. In addition, a contact hole 30 is provided in the position corresponding to an auxiliary capacitor electrode 18a to form an electrical coupling between the pixel electrode 20 and drain electrode D. The structure of this portion will be described in detail below using FIG. 14 and FIG. 15.
On the color filter substrate 14 there are usually provided a black matrix 21, a color filter 22, and a common electrode 23. The black matrix 21 is provided in a matrix form aligned with pixel regions of the array substrate 13. The color filter 22 is composed of, for example, red (R), green (G), and blue (B), etc., provided in the area enclosed by the black matrix 21. The common electrode 23 is electrically coupled to the electrode on the array substrate 13 and provided so as to cover the color filter. In addition, in the area enclosed by the array substrate 13, color filter substrate 14, and sealing material, there are provided multiple columnar spacers, etc. as required to obtain uniform distances between the substrates, and the liquid crystal 15 is filled.
Next, the manufacturing process for the above-mentioned array substrate 13 of a liquid crystal display device 10D will be described below, with reference to FIG. 14 and FIG. 15. First, as shown in FIG. 14A, a conductive material layer 24 of a prescribed thickness made of aluminum, molybdenum, chromium, or their alloy is formed on the transparent substrate 11. Next, as shown in FIG. 14B, a part of the conductive material layer 24 is etched away by performing patterning using a commonly-known photolithography method, and then there are formed the scan lines 16 extending in the crosswise direction and the auxiliary capacitor lines 18 between these scan lines 16. In FIG. 14B there are shown the gate electrode G that extends from the scan line 16, and an auxiliary capacitor electrode 18a that is formed by widening a part of the auxiliary capacitor line 18. The scan line 16 and auxiliary capacitor line 18 are shown as wiring lines of a multilayer structure composed of aluminum and molybdenum. Although aluminum has the advantage of having a small resistance value, it has the disadvantages of being corrosion-prone and having a high contact resistance to ITO. These disadvantages can be improved by using a multilayer structure with aluminum covered by molybdenum.
Next, as shown in FIG. 14C, a thick insulating layer 25 of a prescribed thickness is formed so as to cover a transparent substrate 11 on which the scan lines 16 and auxiliary capacitor lines 18 have been formed in the preceding process. A transparent inorganic insulating material composed of silicon nitride, etc. is used as a material for the thick insulating layer 25. Because the thickness of the thick insulating layer 25 is related to the insulation performance of the scan line 16 and gate electrode G, it is preferable to be in the range of 2000 Å to 4500 Å, and more preferable to be 2800 Å or more. After that, as shown in FIG. 14D, only the portion of this thick insulating layer 25 directly above the auxiliary capacitor electrode 18a is etched away to form a window portion W.
After completion of the above-described process, a thinned insulating layer 26 is formed so as to cover a transparent substrate 11, as shown in FIG. 14E. Since the thinned insulating layer 26 is formed over the thick insulating layer 25 and the auxiliary capacitor electrode 18a, above which the thick insulating layer 25 is etched away, the scan line 16 and gate electrode G are coated with both the thick insulating layer 25 and the thinned insulating layer 26. Thus, this double-layered film constitutes a first insulating film (called also a gate insulating layer) 27 with a thickness of 2500 Å to 5500 Å. Here, the auxiliary capacitor electrode 18a is coated with only the thinned insulating layer 26. The material of the thinned insulating layer 26 may be either the same as that of the thick insulating layer 25, that is silicon nitride, or that of another insulating layer, for example silicon oxide. Its thickness needs only to be thinner than that of the thick insulating layer 25, preferably from 500 Å to 1500 Å, and more preferably approximately 1000 Å, for example from 800 Å to 1200 Å.
Next, as shown in FIG. 14F, on the thinned insulating layer 26 is formed a silicon layer, for example an a-Si layer, with a thickness of 1800 Å, and on its surface is formed an ohmic contact layer (not shown in the drawing) composed of an n+a-Si layer to a thickness of 500 Å. Then, as shown in FIG. 14G, the a-Si layer and n+a-Si layer are etched away, except the portion covering the gate electrode G, to form a semiconductor layer 19 that constitutes a part of TFT.
Then, using the same method as the above, a conductive material film is formed on the transparent substrate 11, and as shown in FIG. 15A, patterning is performed to form the signal lines 17 that extend in a direction perpendicular to the scan line 16, the source electrode S that is extended from the signal line 17 and coupled to the semiconductor layer 19, and the drain electrode D that covers over the auxiliary capacitor electrode 18a and has an end coupled to the semiconductor layer 19. In this way, the TFT serving as a switching element is formed in the neighborhood of the intersection between the scan line 16 and signal line 17 on the transparent substrate 11.
Furthermore, as shown in FIG. 15B, on the transparent substrate 11 so as to cover these wiring lines, a second insulating film (called also a protective insulating film, or passivation film) 28, which is made of inorganic insulating material to stabilize the surface, is formed. Then, as shown in FIG. 15C, an interlayer 29, which is made of organic insulating material to flatten the surface of the array substrate 13, is formed.
Next, as shown in FIG. 15D, the interlayer 29 and the second insulating film 28 are removed at a portion directly above the auxiliary capacitor electrode 18a of the interlayer 29, to form the contact hole 30. Finally, after this step, as shown in FIG. 15E, in each pixel region enclosed by the scan lines 16 and signal lines 17 is formed the pixel electrode 20 made of, for example, ITO or IZO. At this point, it is preferable to provide the pixel electrode 20 so that a part thereof is located on the scan line 16 and signal line 17, and the neighboring pixel electrodes 20 are not in contact with each other. With the above process, manufacture of the array substrate 13 is completed.
The auxiliary capacitor of the array substrate 13 formed by the above-described manufacturing method has a capacitor structure in which the lower electrode corresponds to the auxiliary capacitor electrode 18a, the upper electrode to the drain electrode D coupled to the pixel electrode 20, and the dielectric body to the thinned insulating layer 26. Therefore, since the layer functioning as the dielectric body is an insulating layer with a thickness of 500 Å to 1500 Å, which is thinner than the gate insulating film with the thickness of 2500 Å to 5500 Å functioning as the dielectric body in conventional technology, the capacitor capacitance can be dramatically increased. In addition, since the gate electrode G and scan lines 16 are coated with the first insulating film 27 having a double-layered structure with a thickness of 2500 Å to 5500 Å, which is constituted by the thick insulating layer 25 and the thinned insulating layer 26, the insulation performance is sufficiently ensured.
According to the liquid crystal display device 10D of the above-described prior invention, the capacitor capacitance is increased by reducing the thickness of the thinned insulating layer 26 acting as a dielectric body of a capacitor, enabling the size reduction of the electrode portion that constitutes the auxiliary capacitor. As a result, the aperture ratio of pixels can be increased. In addition, because the drain electrode D works also as an electrode constituting the auxiliary capacitor, the light-blocking portion in the pixel can be made smaller compared with the case in which, in addition to the drain electrode, a special electrode (conductive layer) is provided as an electrode constituting the auxiliary capacitor, thus producing the advantage of further improvement in aperture ratio.
However, according to the liquid crystal display device 10D of the above-described prior invention, the contact hole 30 is provided so as to lie directly above the auxiliary capacitor electrode 18a. The contact hole 30 is formed by etching the interlayer 29 and the second insulating film 28 lying over the drain electrode D, using a dry etching method (plasma etching method). In that case, the drain electrode is subject to pinholes by plasma damage. Moreover, because the thickness of the thinned insulating layer 26 lying under the drain electrode D is as thin as 500 Å to 1500 Å, the portion forming the contact hole 30 is prone to short-circuit between the drain electrode D and auxiliary capacitor electrode 18a, leading to the occurrence of bright point defects. This tendency has been causing the reduction in fabrication yield.